@samueljohn `llvm-gcc4.2` does contain an OpenMP implementation, but it is has some long standing (old) bugs that can cause a build to fail. It is certainly possible that defaulting to `llvm` (because of the `fails_with`) could build fine.  I looked at the source briefly, the makefile defaults to the GNU compiler and appears to use the correct build flags for OpenMP. If no errors popup during `make`, it is probably good. 

Another thing, it's strange that OpenMP is forced in this program, but that is something decided upstream. I would have expected it to have some serial capability, too, but maybe it is too slow to consider.